Distillation, a method of separating the components of a solution, depends on the distribution of the components between a gas phase and a liquid phase. The gas phase is produced from the liquid phase by vaporization. A new liquid phase of different composition is produced from the gas phase by condensation.
Continuous distillation is a relatively simple and commonly employed method to separate volatile components of a mixture based upon differences in their boiling points. Continuous distillation is conceptually composed of many ideal stages of distillation, each of which brings together a liquid phase and a gas phase into intimate contact. The components of the liquid phase and gas phase redistribute by condensation and vaporization, and a new liquid phase and new gas phase of different composition leaves the stage. While the change in composition of one stage may be slight, the use of many stages in a countercurrent arrangement can effect a great overall change in composition, and essentially pure component products can result.
Continuous distillation is routinely performed using columns adapted to facilitate contact of the liquid phase and the gas phase by increasing the condensation surface area. For example, columns have incorporated distillation fingers or internals such as sieve plates, bubble caps, and tray valves, which are easily seen as an approximation of an ideal stage. Column internals such as random packing or structured packing do not have clearly defined ideal stages, but are commonly employed for continuous distillation.
The classical configuration for a continuous distillation process is a distillation column with a single condenser overhead and a single reboiler at the bottom. The feedstock is introduced into a nominal middle of the column. The higher boiling components are removed from the reboiler stage as the bottoms product. The lower boiling components are removed from the condenser stage as the distillate. A portion of the distillate is returned (i.e., refluxed) to the top of the column. The reflux liquid contacts the vapors from the reboiler throughout the column to effect the separation. The technology for sizing and designing continuous distillation is well known. Each design is specific to the components that are being separated and to their unique volatility.
In the case of a mixture of N,N-dimethylacetamide (DMAc) and water, separation of the two pure components is simple because of the large difference in volatility, as can be expected from the boiling point for each pure component. The boiling point at atmospheric pressure for DMAc is 166° C. The boiling point at atmospheric pressure for water is 100° C. In a continuous distillation of a DMAc and water feedstock, water will be the distillate from the condenser and DMAc will be the bottoms product from the reboiler.
Acetic acid is often present as a contaminant in the DMAc and water feedstock. This may be because DMAc can degrade to form acetic acid and dimethylamine (DMA). Alternately, acetic acid may be present from a chemical synthesis where DMAc was used as a solvent.
A mixture of DMAc and acetic acid, however, is not easily separated into component parts, even though the boiling point of acetic acid, 118° C., is sufficiently distinct from the boiling point of DMAc. This is due to a hydrogen bonding effect, whereby DMAc, in the presence of acetic acid, acts as a base and exerts a strong attraction for acetic acid. DMAc and acetic acid form a high boiling azeotropic mixture, defined as a solution of two or more liquids, the composition of which does not change upon distillation. More specifically, the azeotrope mixture of 21% acetic acid and 79% DMAc has a boiling point of about 171° C. at atmospheric pressure.
A classical continuous distillation configuration cannot separate mixtures that form azeotropes into two pure component streams. One of the products of the distillation column will approach the azeotrope composition. A high boiling azeotrope, if it occurs, will concentrate in the bottoms product of the reboiler. A low boiling azeotrope, if it occurs, will concentrate in the distillate product of the condenser.
When a mixture of water, DMAc, and acetic acid is fed into a classical distillation column configuration, acetic acid will separate into the bottom of the column, and thereby contaminate the DMAc, because acetic acid will form a high boiling azeotrope with DMAc.
Attempting to remove DMAc as a side-draw vapor stream at or near the bottom of the column is not an effective solution. Acetic acid will eventually build to concentrations in the bottom of the column that approach the DMAc-acetic acid azeotrope. The vapors of the azeotrope will migrate up the column and contaminate the side-draw DMAc vapor stream. The concentration of acetic acid in the bottom of the column could be controlled to a concentration well less than the azeotrope by purging the bottoms of the column. However, this will undesirably result in a large loss of DMAc. Redistilling the bottoms DMAc product to remove DMAc as an overhead product in a second distillation column would require the bottoms of the second column to be purged to control acetic acid below its azeotrope composition, again resulting in substantial loss of the desired DMAc component.
French Patent No. 1,549,711 discloses that the bottoms product of a second column, being a feedstock of DMAc and acetic acid, essentially dry of a third component water, could be fed to a third column operated at a different pressure to change the azeotrope composition. This, however, involves the additional expense and step of a third column. Moreover, the system is not disclosed for separation of DMAc, water, and acetic acid.
A method to overcome the aforementioned problems is therefore desired.